Operation of lean burn engines, for example diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy and have very low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Diesel engines, in particular, offer significant advantages over gasoline engines in terms of their fuel economy, durability, and their ability to generate high torque at low speed. From the standpoint of emissions, however, diesel engines generally exhibit more severe problems than their spark-ignition counterparts. Emission problems relate to particulate matter, nitrogen oxides (NOx), unburned hydrocarbons (HC) and carbon monoxide (CO), wherein NOx describes various chemical species of nitrogen oxides, including nitrogen monoxide and nitrogen dioxide, among others.
Oxidation catalysts comprising precious metals, such as gold, platinum, palladium, rhodium, iridium, ruthenium and osmium, dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to convert both hydrocarbon and carbon monoxide gaseous pollutants by catalyzing the oxidation of these pollutants to carbon dioxide and water. Such catalysts may be contained in diesel oxidation catalysts (DOC) or, more generally, catalytic converters which are placed in the exhaust flow path from a diesel powered engine to treat the exhaust gas stream. Typically, the diesel oxidation catalysts are prepared on ceramic or metallic carrier substrates upon which one or more catalyst coating compositions are deposited. In addition to the conversion of gaseous HC, CO and the soluble organic fraction of particulate matter, oxidation catalysts containing precious metals dispersed on a refractory oxide support may promote the oxidation of nitric oxide to nitric dioxide.
As is well-known in the art, catalysts used to treat the exhaust of internal combustion engines are less effective during periods of relatively low temperature operation, such as the initial cold-start period of engine operation, because the engine exhaust is not at a temperature sufficiently high for efficient catalytic conversion of noxious components in the exhaust. To this end, it is known in the art to include an adsorbent material, which may be a zeolite, as part of a catalytic treatment system in order to adsorb gaseous pollutants, usually hydrocarbons, and retain them during the initial cold-start period. As the exhaust gas temperature increases, the adsorbed hydrocarbons are driven from the adsorbent and subjected to catalytic treatment at the higher temperature.
As mentioned, oxidation catalysts comprising a precious metal dispersed on a refractory metal oxide support are known for use in treating exhaust gas emissions from diesel engines. Platinum remains the most effective platinum group metal for oxidizing CO and HC in a DOC, after high temperature aging under lean conditions and in the presence of fuel sulfur. Nevertheless, one of the major advantages of using palladium based catalysts is the lower cost of palladium compared to platinum. However, palladium based DOC typically show higher light-off temperatures for oxidation of CO and HC, especially when used with HC storage materials, potentially causing a delay in HC and or CO light-off. Palladium containing DOC may decrease the activity of platinum to convert paraffins and/or oxidize nitric oxide and may also make the catalyst more susceptible to sulfur poisoning. These characteristics have typically prevented the use of Pd as an oxidation catalyst in lean burn operations especially for light duty diesel applications where engine temperatures remain below 250° C. for most driving conditions.
WO 2010/083313 discloses a diesel oxidation catalyst comprising at least two, specifically three distinct layers, at least one of which contains an oxygen storage compound that is present in a layer separate from the majority of the precious metal components, such as palladium and platinum. The catalyst disclosed therein aims to improved light-off performance.
EP 1 938 893 A2 discloses an engine exhaust catalyst comprising a first supported catalyst comprising platinum and a second supported catalyst comprising palladium and gold. The catalyst is directed to the reduction of CO emissions from a vehicle.
However, as emissions regulations become more stringent, there is a continuing goal to develop diesel oxidation catalyst systems that provide improved performance, for example, light-off performance. Consequently, it was an object of the present invention to provide a diesel oxidation catalyst which shows improved CO conversions and light-off temperatures.
Surprisingly, it was found that this object can be solved by a diesel oxidation catalyst which comprises a washcoat layer which is disposed on a carrier substrate, wherein in this washcoat layer, palladium, gold, and additionally a ceria comprising compound are contained, wherein palladium is supported on a support material which comprises a metal oxide and gold is supported on a support material comprising a metal oxide.